Automatic fidelity control



Patented Apr. 20, 1937 UNITED STATES PATENT OFFICE Radio Corporation ofDelaware America, a corporation of Application November 28, 1933, SerialNo. 700,034

7 Claims.

My present invention relates to control circuits for radio receivers,and more particularly to a radio receiver including automatic fidelityconrol. i

It is highly desirable tohave a radio receiver, particularly oneoperating in a broadcast range, possess a variable audio responsecharacteristic. For example, the radio receiver should have a broaderaudio response characteristic when receiving strong local stations thanwhen receiving relatively weaker distant stations. In the case of thereception of relatively weak signals from distant stations, it is ofconsiderable advantage to substantially reduce the receiver response tothe higher audio frequencies since the background noises reside in thehigher audio frequency portion of the audio range.

Various arrangements have been proposed heretofore for securingautomatic fidelity, or selectivity, control in a radio receiver. In manyof these arrangements the resonance curve characteristic of a tunedcircuit feeding the audio network is varied according to the change inreceived carrier amplitude. One of the problems encountered in thepractical use of such systems is the increase of interference from anadjacent unwanted station frequency.

Accordingly, it may be stated that it is one of the main objects of thepresent invention to provide in a radio receiver an automatic fidelitycontrol tube which feeds a tuned network coupled to the audio network ofthe receiver, and wherein a variable audio response having betterfidelity on local stations than on distant stations is obtained, while,at the same time, there is not secured an increase in the interferencefrom an adjacent station frequency above a given amount determined bythe set designer. Another important object of the present invention isto provide a radio receiver which not only includes an automatic volumecontrol arrangement for rendering the intensity of the carrier amplitudefed to the detector substantially constant, but also includes anautomatic fidelity control arrangement which functions to vary theoverall selectivity curve of a receiver, in such a manner that thereceiver more faithfully reproduces signals from a strong local stationthan from a relatively weaker distant station.

Still another object of the invention is to provide in a superheterodynereceiver provided with an intermediate frequency amplifier feeding thereceiver detector through a tuned network an improved type of fidelitycontrol tube in the amplifier whose plate impedance is automatically(Cl. Z50-20) regulated in accordance with the variations in the receivedcarrier amplitude, so that the control tube has a minimum plateimpedance when receiving strong signals and a maximum plate impedancewhen receiving relatively weaker signals.

Still other objects of the invention are to improve generally thesimplicity and efficiency of broadcast radio receivers, and toparticularly provide such a receiver with automatic volume control andautomatic fidelity control arrangements which are not only reliable inoperation, but add no control elements to be operated manually by thebroadcast receiver listener.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,will best be understood by reference to the following description, takenin connection with the drawing, in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

In the drawing- Fig. 1 shows diagrammatically a receiving systemembodying the present invention,

Fig. 2a. shows the resonance curve characteristic of the firstintermediate frequency amplifier, which is predetermined by the setdesigner,

Fig. 2b graphically shows the resonance characteristics of the secondintermediate frequency amplifier, as a result of the automatic control,

Figs. 3 and 4 graphically illustrate the overall selectivity curve ofthe receiver of Fig. 1 when receiving weak and strong signalsrespectively.

Referring now to the accompanying drawing, wherein like referencecharacters in the different figures designate similar elements, there isshown in Fig. 1 a radio receiving arrangement, which arrangement isshown as of the superheterodyne receiver type, for the purpose ofillustrating the present invention. The signal collector comprises theusual grounded antenna circuit l, and this circuit is connected to atunable radio frequency amplifier, which is conventionally represented.Those skilled in the art are well acquainted with the construction ofthe radio frequency amplifier, and it is preferred that thisconstruction be similar to present-day construction. The output of theradio frequency amplifier is impressed upon a combined first detectorand tunable local oscillator` circuit. Of course, the first detector andoscillator network can either employ two separate tubes, one of thetubes functioning as the first detector and the other tube as the localoscillator, or a single tube functioning as a combination detector andoscillator may be utilized. Both of these arrangements are well known tothose skilled in the art at the present time, and, therefore, furtherexplanation is not believed necessary. If a combined first detector andlocal oscillator circuit is employed, there may be used for such acircuit a 2A? tube (pentagrid converter tube), as disclosed. by Hainesapplication Serial No. 653,171, filed March 28, 1933.

The intermediate frequency energy of the first detector is thenimpressed upon an intermediate frequency amplifier, which is tuned to anintermediate frequency of 1'75 kilocycles. This first intermediatefrequency amplifier is a band pass amplifier, and its resonance curvecharacteristic is illustrated in Fig. 2a.. The design of the 175 k. c.

band pass amplifier is such that it passes the maximum band of thereceiver (12 k. c.) lat 90% of resonance value. At this` point it ispointed out that one of thel conditions imposed as satisfactoryoperation is that the overall selectivity of the receiver must vary from4 kilocycle band width at 901% of resonance value to 12 kilocycle bandwidth, with substantially the same interference ratio at, for example,20 kilocycles off resonance. Hence, the resonance of curvecharacteristic in Fig. 2a shows that the first intermediate frequencyamplifier is designed so that at 20 kilocycles off resonance it givesthe desired interference ratio. The output of the 175 k. c. amplifier isfed into a second detector network. This network, while shown as acombined second detector and local oscillator network, may employ twoseparate tubes, one of them functioning as the second detector, whilethe other functions as a local oscillator. In either case, the localoscillator is fixed, and not variable, as in the case of the firstdetector network. The second detector network changes the frequency ofthe output from the first intermediate frequency amplifier to a secondlower intermediate frequency of a value of, for example, 40 to 50kilocycles.

This second intermediate frequency is introduced into a secondintermediate frequency amplifier through a resonant network 2, which istuned to the second intermediate frequency of 40 kilocycles. The secondintermediate frequency amplifier utilizes a tube 58 of the pentode type.That is to say, the tube 58, besides including the usual signal grid,cathode, positive screen grid and anode, also includes a suppressor gridbetween the screen grid and anode.

The suppressor grid is denoted by the numeral 3, and this tube 58functions as the automatic fidelity control tube. A third detectorfollows the tube 58, and the coupling between the plate of tube 58 andthe input electrodes of the third detector includes a transformer d,which possesses a tuned primary and a tuned secondary both tuned to thesecond intermediate frequency of 40 kilocycles. The amplifier networkincludingtube 58 is designed to have a selectivity curve similai tocurvel of Fig. 2b when the plate impedance of tube 58 is a maximum. Thelatter condition holds true when the voltage of the suppressor grid 3 isat cathode potential. At 90% of resonance the band width is to be 4kilocycles. Now, with minimum plate impedance the selectivity curve ofthis amplifier will assume some such shape as shown by curve 2 of Fig.2b. This latter curve results because the tuned circuits of thetransformer 4 are shunted by the low plate impedance of the automaticfidelity control tube 58.

ance condition of the tube 58.

The detector output of the third detector is fed into an audio frequencyamplifier, and the audio output of the latter is reproduced in any wellknown manner, as by a loud speaker. The control Voltage for the fidelitycontrol function and the volume control function is derived from arectier 5 arranged to have signal energy impressed upon it through asignal path 6 from the input circuit of the third detector. The rectiermay be of the diode or triode type, and in either case the directcurrent component of the rectified signal voltage is employed for thetwo control functions. It is not believed necessary to show the detailedcircuit connections of the rectifier 5 for the reason that itsconstruction is well known to those skilled in the broadcast receiverart at the present time. Reference is made to application Serial No.642,544, filed November 14th, 1933 by Chittick, et al. for anillustration of an arrangement which may be employed to secure thecontrol voltages in this case. i

The automatic volume control pathV includes the direct currentconnection. l between the rectifier 5 and the grid circuits of the radiofrequency amplifier, the first detector and the 175 k. c. in-

termediate frequency amplifier. Such a volume control path customarilyincludes the resistorcondenser filter arrangement 8 for suppressingripples in the volume control voltage. Of course,

the functions of the rectifier 5 and the third detector may be combinedin a single tube, and such a construction is also very well known tothose skilled in the art, the direct current component of the detectorplate current being employed in that case for the volume controlfunction. The

direct current component of the rectified signal voltage of rectifier 5is also employed to control the direct current Voltage of the suppressorgrid 3, and this is accomplished through a lead 9, a radio frequencyfilter ID being connected between ground and the lead 9.

The main limitation of the application of an automatic fidelity controltube, such as tube 58, in a Yradio receiver isv that when the tube hasminimum plate impedance, the selectivity of the tuned circuit acrosswhich it is placed is very materially broadened. This means that theVoltage input from the interfering station, to give the same audiooutput as the wanted station, becomes less. Another way of explainingthis phenomenon is that when the fidelity of the receiver becomes good,less signal strength from an interfering station is required to give thesameV audio output as that from the desired station. It is for thisreason that the condition, stated heretofore, i

was imposed for satisfactory operation; to wit: that the overallselectivity must vary from 4 kilocycle band width at of resonance valueto 12 kilocycle band width with substantially the same interferenceratio at say 20 kilocycles off resonance. The resulting overallselectivity curve of the receiver is arrived at by combining the twoselectivity curves of the second intermediate frequency amplifier withthat of the first intermediate frequency amplifier. These results areshown in Figs. 3 and 4.

In Fig. 3 the shaded portion S shows the overall selectivity or responseof the receiver at maxif mum Rp (plate impedance) of the automaticfidelity tube 58. It will be observed that this curve is obtained bycombining curve I of Fig. 2b with the curve shown in Fig. 2a. In Fig. 4the shaded portion S' shows the overall selectivity of the receiver atthe minimum plate imped- It will be observed that in both cases theinterference ratio at 20 kilocycles off resonance is better than theband pass amplifier alone, the latter being shown in Fig. 2a. One of theconditions imposed was that the band pass amplifier be designed so thatthe interference ratio of it alone was as good as was wanted. It wouldreach this condition as a limit only if there were no selectivity in thesecond intermediate amplifier at minimum plate impedance of tube 53. i

The operation of the receiving arrangement shown in Fig. 1 is asfollowsz-The action is such that when the amplitude of the carrier issmall, as when receiving a distant station, the negative potentialgenerated by the rectifier 5 is small, and the second intermediatefrequency amplifier .tubes plate impedance is high and loading on itstuned plate circuit is small. Therefore, its se-. lectivity .is good.This results in the non-reproduction of the higher frequency side bandsof the received signal. This gives a fidelity characteristic which doesnot reproduce the higher audio frequencies and attendant extraneoushigh-frequency noise which would accompany the reception of a weaksignal. When a strong local signal is received a high negative potentialresults at the rectifier 5, due to a large carrier amplitude. This largenegative potential on the suppressor grid of the second intermediatefrequency amplifier tube will cause the tube plate impedance to bereduced to a low value.

This low impedance causes the associated tuned circuit in the platecircuit of this tube to be severely loaded and results in a poorselectivity curve. As a direct result of this reduced selectivity curvethe audio fidelity becomes good; which is to say the high frequenciesare produced in the audio output. Since the signal is large and producesa large negative voltage at the rectifier, and this negative voltage isused as an automatic sensitivity control on the rest of the receiver, itfollows that the sensitivity is also greatly reduced and the receiverbecomes less susceptible to extraneous noises.

Since the selectivity of the second intermediate frequency amplifier isbroad it follows that its rejectivity to adjacent channel interferenceis also reduced. This feature is undesirable and by use of a firstintermediate frequency amplifier, as before mentioned, whose selectivitycharacteristic is controlled by design to give the desired rejectionratio this undersirable feature is eliminated. This is illustrated bycombining the selectivity curves of the two intermediate frequencyamplifiers which give as a result the overall selectivity characteristicwhich the third, or audio, detector operates on. This is illustrated bythe shaded portions of Figs. 3 and 4 which show the reception of a weakor distant station, and also a strong or local station.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:-

l. In a superheterodyne receiver, an intermediate frequency amplifier, adetector, a resonant network coupling said amplifier and said detector,said network being tuned to the operating intermediate frequency, meansfor automatically controlling the direct current potential of anelectrode of said amplifier to vary the plate impedance thereofsufciently to regulate the selectivity of said network, said amplifiercomprising a pentode tube provided with a suppressor grid between itsscreen grid and anode, and said potential varying means including adirect current connection to the suppressor grid.

2. In combination, in a superheterodyne receiver, an intermediatefrequency amplifier having a band pass characteristic, said amplifierbeing designed to pass a maximum band of 12 kilocycles at of resonancevalue, a second detector and a following second intermediate frequencyamplifier tuned to a frequency substantially less than the frequency ofthe first amplifier, said second amplier means designed to have a bandwidth of 4 kilocycles at 90% of resonance, a rectifier following saidsecond amplifier, and a direct current connection between a coldelectrode of said second amplifier and said rectifier for varying theplate impedance of said second amplifier in such a manner that theoverall selectivity of the receiver is sharper when receiving weaksignals than when receiving strong signals.

3. In a superheterodyne receiver including two successivefrequencychanger circuits for reducing received signals of a frequencyfrom 550 to 1500 kilocycles to a frequency of the order of 40kilocycles, a detector, an amplifier between the said detector and thepreceding frequency changer circuit, said amplifier including a pentodetube provided with a suppressor grid between the screen grid and platethereof, a rectifier coul pled to the plate of the pentode amplierthrough a tuned network to receive signal energy therefrom, and a directcurrent connection from the rectifier to said suppressor grid to varythe amplifier plate impedance and thereby regulate the tuned networkselectivity.

4. In a radio receiver of the superheterodyne type, at least twosuccessive amplifiers, each amplifier being tuned to an operatingcarrier frequency, the second amplifier frequency being substantiallylower than the first amplifier frequency, a network, tuned to the secondamplifier frequency, coupled to the second amplifier, and means,responsive to received carrier amplitude variations, for varying theoutput impedance of the second amplifier in such a manner that thesharpness of said tuned network is varied in a sense opposite to saidamplitude variations.

5. In a superheterodyne receiver including two successive frequencychanger circuits for reducing received signals of a frequency from 550to 1500 kilocycles to a frequency of 40 kilocycles, a detector, anamplifier between the third detector and the preceding frequency changercircuit, said amplier including a pentode tube provided with asuppressor grid between the screen grid and plate thereof, a rectifiercoupled to a point in the receiver system to receive signal energytherefrom, a direct current connection from the rectifier to saidsuppressor grid, and an additional direct current connection between therectifier and a point in the receiver system preceding said amplifier.

6. In a radio receiver of the superheterodyne type, at least twosuccessive amplifiers, each amplier being tuned to an operating carrierfrequency, the second amplifier frequency being substantially lower thanthe first amplifier frequency, a network, tuned to the second amplifierfrequency, coupled to the second amplifier, and means, responsive toreceived carrier amplitude cluding a pentode tube provided with asuppressor grid between the screen grid and plate thereof, and means,responsive to received signal amplitude changes, connected to thesuppressor grid to vary the selectivity of said network in a senseopposite to said amplitude changes, and means, responsive to receivedsignal amplitude Variations, for automatically controlling the signalinput to said amplier in such a manner that the signal amplitude at theamplier input circuit is substantially constant over a relatively Widerange 10 of received signal amplitude variation.

LOUIS C. HOLLANDS.

